This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section. Acronyms used in the drawings and this disclosure are defined at the end of this disclosure.
Regarding small cells enhancements, two points of interest are dual connectivity and bearer split. Dual connectivity is introduced because of the non-ideal backhaul of a small cell, where Pcell and Scell operation separated by such a backhaul could previously not be supported, which introduces the concepts of Master eNB (MeNB) and Secondary eNB (SeNB).
Dual connectivity concerns an operation where a given UE consumes radio resources provided by at least two different network points (Master and Secondary eNBs) connected with non-ideal backhaul while in RRC_CONNECTED. A bearer split in dual connectivity refers to the ability to split a bearer over multiple eNBs.
A master cell group (MCG) is the group of the serving cells associated with the MeNB. A Master eNB, in dual connectivity, is the eNB which terminates at least S1-MME and therefore acts as mobility anchor towards the CN. A secondary cell group (SCG) is the group of the serving cells associated with the SeNB. A Secondary eNB (SeNB) in dual connectivity is an eNB providing additional radio resources for the UE, which is not the Master eNB. And Xn is interface between MeNB and SeNB. If the current E-UTRAN architecture was selected as baseline, Xn would mean X2.
3GPP RAN2 and RAN3 have been working on Dual Connectivity work item (RP-132069). In general terms there are three types of bearers of dual connectivity known in the art. For MCG bearers, the MeNB is U-plane connected to the S-GW via S1-U, where the SeNB is not involved in the transport of user plane data. For split bearers, the MeNB is U-plane connected to the S-GW via S1-U and in addition, the MeNB and the SeNB are interconnected via X2-U. For SCG bearers, the SeNB is directly connected with the S-GW via S1-U.
Particularly for split bearers, one open issue being discussed in RAN3 is how flow-control data requests are defined, particularly signaling indications by which SeNB indicates to MeNB how much more downlink data for a given UE MeNB should send via SeNB.
The draft stage-3 TS capturing the agreed indications is in R3-142037 discussing successful operation of Downlink Data Delivery Status. The purpose of the Downlink Data Delivery Status procedure is to provide feedback from the SeNB to the MeNB to allow the MeNB to control the downlink user data flow via the SeNB for the UE and its individual split E-RABs. The SeNB may also transfer uplink user data for the concerned E-RAB to the MeNB together with a DL data delivery status frame within the same GTP-U PDU. When the SeNB decides to trigger the Feedback for Downlink Data Delivery procedure it shall report the following:
(a) the highest PDCP PDU sequence number successfully delivered in sequence to the UE among those PDCP PDUs received from the MeNB;
(b) the available buffer size in bytes for the concerned E-RAB, counted from the PDCP PDU sequence number reported under (a) above; and
(c) the available buffer size in bytes for the UE, counted from the PDCP PDU sequence number reported, as described under (a) above, for the concerned E-RAB and most recently reported for all other E-RABs established for the UE.